The optical pumping structure is a subassembly that is critical to laser performance.
It requires good extraction of the thermal power dissipated into the active medium but also a good uniformity of the temperatures of the pumping diodes used, in order to ensure a uniform distribution of the gain of the laser during a firing sequence and at different rates between two consecutive pulses in the case of a pulsed laser. It will be recalled that for a pulsed laser, the average power dissipated by the structure, Pavg, is given by the relationship: Pavg=Pdiodes×nb diodes×pumping time×pulse rate, Pdiodes being the power of each set of pumping diodes.
The rate and the duration of the pumping linked to the duration of the firing sequence therefore vary the dissipated power which has an impact on optical performance.
There are currently pumping structures with a rectangular configuration, a cross-sectional view of which is shown in FIG. 2a. Such a structure comprises a longitudinal rod 1 with a square cross section used as the active medium; it is pumped by two sets of diodes 2 arranged along the rod and opposite two parallel faces, the two other faces of the rod being in contact with an element for extracting heat 3 by conduction. While such a configuration is effective for the thermal dissipation, it is not so effective with respect to the quality of the beam obtained at the output: the figure of the output beam 10 shown in cross-section in FIG. 2b, imposed by the geometry of the section, is not circular.
Another solution involves using a pumping structure with circular configuration, that is to say, comprising a rod 1 with circular cross-section; a cross-sectional view is shown in FIG. 1a. The figure of the output beam 10, imposed by the geometry of the section and shown in FIG. 1b, is then circular which ensures a good quality beam. Sets of pumping diodes 2, for example three sets, are arranged in the form of a star around the rod 1. Thermal dissipation elements in the form of aluminum rings 4 grip the rod 1 at its ends as can be seen in FIG. 1c; the thermal dissipation is also provided by a cooling device 3 that uses a circulation of liquid which links the sets of diodes 2 as can be seen in cross-section in FIG. 1a. An example of such a pumping structure with liquid cooling and comprising four sets of diodes arranged in the form of a star is presented in the U.S. Pat. No. 6,101,208, which also indicates that cooling by conduction does not give satisfaction. In this case, the good quality of the beam is obtained to the detriment of a significant bulk and the use of a coolant.
An effective cooling using a gas (air for example) instead of a liquid in a minimum bulk is a major issue.
Consequently, there remains, to this day, a need for a pumping structure that simultaneously satisfies all the abovementioned requirements, namely a cooling by gas, a good extraction of the thermal power dissipated into the active medium (reduction of the temperatures and of the axial and longitudinal internal thermal gradients), a good uniformity of the temperatures of the pumping diodes used, a minimum bulk and a good quality of the optical beam.